Narrated AbuAyyub al-Ansari:
The Prophet (peace be upon him) said: The witr is a duty for every Muslim so if anyone wishes to observe it with five rak'ahs, he may do so; if anyone wishes to observe it with three, he may do so, and if anyone wishes to observe it with one, he may do so.
Sunan Abi Dawud, Book 8, Hadith 1417